In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric and water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The paper machine is, in essence, a water removal system. A typical forming section of a papermaking machine includes an endless traveling papermaking fabric or wire, which travels over a series of water removal elements such as table rolls, foils, vacuum foils, and suction boxes. The stock is carried on the top surface of the papermaking fabric and is de-watered as the stock travels over the successive de-watering elements to form a sheet of paper. Finally, the wet sheet is transferred to the press section of the papermaking machine where enough water is removed to form a sheet of paper. Many factors influence the rate at which water is removed which ultimately affects the quality of the paper produced.
It is well known to continuously measure certain properties of the paper material in order to monitor the quality of the finished product. These on-line measurements often include basis weight, moisture content, and sheet caliper, i.e., thickness. The measurements can be used for controlling process variables with the goal of maintaining output quality and minimizing the quantity of product that must be rejected due to disturbances in the manufacturing process. The on-line sheet property measurements are often accomplished by scanning sensors that periodically traverse the sheet material from edge to edge.
It is conventional to measure the caliper of sheet material upon its leaving the main dryer section or at the take-up reel with scanning sensors. Such measurements may be used to adjust the machine operation toward achieving desired parameters. Numerous methods exist for measuring the thickness of a moving web or sheet, such as paper. Two of the most common techniques include direct thickness measurements using contacting glides or shoes, which skim along the two surfaces of the web, and non-contacting inferential method in which radiation absorption by the web is used to determine the weight per unit area of the web and the thickness is thereafter inferred, provided the density of the material is known with sufficient precision. Many variations and improvements to these methods exist, but each of the techniques has underlying drawbacks.
The contacting method is subject to three fundamental types of problems. First, the method can be limited by the strength of the material being measured. With fragile sheets such as tissue, for example, there is a tendency for the contacting shoes to snag deviations in the sheet surface, causing flaws in the sheet or even causing the sheet to tear. Second, the sheet itself can damage a contacting caliper sensor due either to abrasive wear on the contacting elements or to physical damage arising during sheet breaks. For caliper sensors that traverse the sheet, damage can also be caused when the sensor crosses the sheet edge. Third, the accuracy of contacting sensors can be adversely affected by the buildup of contaminants on the contacting elements, as may occur with coated or filled sheets or sheets containing recycled materials.
The non-contacting inferential thickness measurement methods avoid many of the problems of the contacting methods but are subject to a new set of problems. Several patents have suggested that use of lasers to measure the thickness of a moving web may be a promising option compared to the other methods available. One such system is described in U.S. Pat. No. 5,210,593 to Kramer and another is described in U.S. Pat. No. 4,276,480 to Watson. In both systems, the laser caliper apparatus comprises a laser source that is positioned on both sides of the web whose light is directed onto the web surface and subsequently reflected to a receiver. The characteristics of the received laser signal are thereafter used to determine the distance from each receiver to the web surface. These distances are added together and the result is subtracted from a known value for the distance between the two laser receivers. The result represents the web's thickness.
The above non-contacting approaches to thickness measurements have the desirable feature of eliminating many of the disadvantages of the contacting method and the non-contacting inferential methods. However, there are difficulties with previous non-contacting techniques that can limit their usefulness to relatively low-accuracy applications.
One of the problems is that the web may not always be perpendicular to the incident light since the web has a tendency to bounce or develop intermittent wave-like motion. If the web is non-perpendicular to the incident light and the light beams from two opposing light sources are not directed to exactly the same spot on the sheet, substantial error in measurement can occur. This is caused by a number of factors. First, actual web thickness variations from the first laser's measurement spot to the second laser's measurement spot can cause an incorrect thickness measurement. Second, if the web is not perpendicular to the incident light, the measurement technique will cause an error in the thickness value proportional to the web's angle and to the displacement on the sheet surface between the two measurement spots. Bouncing or oscillation of the web can further exacerbate this error.
U.S. Pat. No. 6,281,679 to King et al. describes a non-contact web thickness measurement system which has distance determining means on opposite sides of the web. The system includes a caliper sensor that is capable of accurate on-line web thickness measurements even when continuously scanning the system across the web. The air clamp can be operated such that air flow will force the machine direction moving sheet to a minimum displacement position as seen by the laser underneath the air clamp (and a maximum for the other). The air clamp is largely designed such that the sheet displacement is largely invariant in the cross direction. At this position, small x-y displacements introduce minimal error to the measurement. This assumes that sheet thickness is largely determined by paper machine properties and that the microstructure of the paper is not considered. This is true when average measurements are made which is generally the case for practical measurements on moving sheets.
When employed to measure the thickness of paper, the caliper sensor is typically stationed at the dry end of the papermaking machine. It has been assumed that the sheet's surface is perpendicular to the laser beams and relatively flat along the machine direction in the region where the paper passes through the caliper sensor so that the ideal interrogation spot is directly above and below the upper and lower sensor heads respectively. However, it has been discovered that the sheet's surface is not sufficiently planar. The result was that the ideal interrogation spot is often located on a part of the surface of the sheet that did not yield precise thickness measurements. Moreover, it is difficult to change the location of the interrogation spot since that would entail disassembling and physically moving the caliper sensor either upstream or downstream from its original position.